Among the most common pesticides are many compounds whose mode of action is presumed to result from interference with the functions of acetylcholine. These include the organophosphate and carbamate cholinesterase inhibitors which are widely used as nematicides and insecticides. These compounds are toxic to many organisms other than their intended targets, including man. They are common environmental toxins. In order to evaluate the use of these pesticides, we need to understand their mode of action in detail. This requires a precise knowledge of the functions of acetylcholine in the target organisms and in other organisms that may be contacted by the pesticides. We have previously investigated the localization of enzymes which synthesize and degrade acetylcholine (ACh) within tissues and single nerve cells of Ascaris, a large parasite. We have shown that choline acetyltransferase (CAT) is localized in three classes of excitatory motorneurons (section A.e.b.). Thus ACh appears to be the excitatory neurotransmitter to Ascaris muscle. However, we have also found that CAT is selectively localized in hypodermis (section A.3.b.) and in gonadal tissue (section A.3.b.) in amounts which are much larger than those found in neurons. (Neuronal CAT represents less than 1% of the total CAT activity). Cholinesterase (ChE) activity within separated tissues of Ascaris has been found to be most concentrated in the intestine and in somatic muscle (section A.3.b.). Other tissues (hypodermis, the nervous system and gonads) have lesser amounts. In muscle there are two forms with different kinetic properties and sedimentation coefficients. The distribution of these forms in different regions of mucle is not uniform, rather the smaller form predominates in anterior muscle whereas in posterior muscle the large form is in excess.